1. Field of the Invention
The present invention relates to an image reproducing method and, more particularly, to a multiplex image reproducing method of superposing toner images on an image retainer having a photoconductive photosensitive member by repeatedly retaining and developing electrostatic images on the image retainer.
2. Description of the Prior Art
As the above-specified image reproducing method, there are known in the art the methods which are disclosed in Japanese Patent Laid-Open Nos. 144452/81, 116553/83 and 116554/83.
In any of these methods, the image retainer has on its surface a layer of a photoconductive photosensitive material such as Se, and the development resorts to a reversal method in which there is applied to an electrostatic image having a lower potential than that of the background a toner for frictionally charging it with the same polarity. According to this method, there arises a problem that the photoconductive photosensitive surface layer is generally and relatively liable to have its electrostatic image retaining performance changed by the charging step and to be subjected to the "toner filming" or have its photosensitivity degraded. As compared with such a positive developing method as in the ordinary electrophotographic reproducing machine, i.e., the developing method in which the electrostatic image has a higher potential than the background so that a toner charged with an opposite polarity is applied to that electrostatic image, moreover, the development by the reversal developing method has a problem that control of the toner application is so difficult that a sufficient development density cannot be attained or that a reproducing apparatus is liable to have its inside blotted by the toner scattered.
As the method in which an image retainer having a dielectric surface layer is used to retain an electrostatic image on the dielectric surface layer, on the other hand, there is known in the art a method using an electrostatic recording head, a method using a screen photosensitive member (as is disclosed in Japanese Patent Publication No. 34616/79) or a method using a screen control electrode (as in disclosed in Japanese Patent Laid-Open No. 137363/81). The electrostatic image retaining methods thus disclosed are superior in that the electrostatic image retainability and the toner image retainability required of the photosensitive member are separated of each other. In these methods, more specifically, it is deemed that the share of retaining the electrostatic image is borne by the electrostatic recording head, the screen photosensitive member or the screen control electrode, whereas the share of retaining the toner image is borne by the dielectric surface layer. Those methods are featured by that color data are retained consecutively and independently as the electrostatic image on the dielectric surface layer. However, the toner image formed on the dielectric surface layer cannot be other than a monochromatic one at all times.
This is because the developing method is conducted by the contact development so that a previous toner image is disturbed or color mixing occurs upon the development even if another electrostatic image could be recorded on the previously formed toner image.
As the method in which an image retainer having a magnetic surface layer is used to form a magnetic image on the magnetic layer, on the other hand, there is known in the art methods which are disclosed in Japanese Patent Laid-Open Nos. 90342/75, 100732/76 and 106253/81. These reproducing methods resorting to that magnetic image is excellent in that the retainability of an electrostatic image by the corona discharge or the like and the retainability of a toner image required of the photosensitive member can be separated from each other. In the reproducing methods using the magnetic image, more specifically, it is deemed that retention of the magnetic image makes whole use of the inside of the magnetic layer while not having its surface state changed as is different from the photosensitive member, and that the share of the toner image retainability is borne by the surface of the magnetic layer. Those methods are featured by that the color data can be newly retained as the magnetic image independently of the toner image formed on the magnetic surface layer. However, the toner image retained on the magnetic layer cannot be other than a monochromatic one at all times.
The method disclosed in Japanese Patent Laid-Open No. 144452/81 retains a color image on an image retainer: by forming an electrostatic image on the surface of an image retainer, which has been charged by a charger, by first exposure means and developing it by first developing means; by forming an electrostatic image on the same charged surface by second exposure means and developing it by second developing means; and by forming an electrostatic image on the same charged surface by third exposure means and developing it by third developing means. The method thus specified has problems that the separate exposure means are required for the respective ones of the repeated formations of the electrostatic images to enlarge the size of the reproducing apparatus and to raise the cost of the same and that synchronizations of the exposure of the respective exposure means with the image retainer have relationships with the respective positions of the exposure means so that the synchronous control is troublesome to make it liable to invite color shift. Moreover, each of the development of that method is conducted by the forced method in which the electrostatic image having a lower potential at its exposed portion than that of the background has such toner applied thereto as is charged with the same polarity. In that forced developing method, the toner for effecting the charge at the same polarity as that of the charge of the image retainer is so used in the developer that it may not be applied to the background. As a result, the reversal developing method has a problem that although the toner is repulsed by the background potential so that it is reluctant to invite any fog, it is also reluctant to be applied to the electrostatic images so that a sufficient developed density can hardly be obtained.
Since the reversed image is obtained, according to this reversal developing method, color reproduction of a positive image cannot be effected so that the coloring is limited to the technique using the dot exposure of a printer or the like. In case it is intended to obtain a positive image when an ordinary original is to be reproduced, it is reversed, and the counter-measure for this reversal is difficult. Since the potential at the photosensitive layer of the exposed portion is at the same polarity as that of the developer although it is low, moreover, the reversal developing method has a problem that the developer is reluctant to be applied to the electrostatic images so that it is liable to be scattered to blot the inside of the reproducing apparatus.
On the other hand, the disclosed in Japanese Patent Laid-Open No. 144452/81 is one conducted under a non-contact jumping developing condition in which the second and later developments by the reversal developing method are conducted such that the layers of the developers formed by the developing means are not in contact with the surface of the image retainer. This method has problems that the development is reluctant to have a sufficient density and is liable to be blotted with the toner scattered unless a strong bias voltage is applied to the developing means to strongly apply the toner to the electrostatic images, and that, the strong bias voltage is applied to the developing means, it is liable to leak to the image retainer or the like, or toner of another color is liable to stick to the toner image developed before or the background.
The methods disclosed in Japanese Patent Laid-Open Nos. 116553/83 and 116554/83 are substantially the same as that in Japanese Patent Laid-Open No. 144452/81 in that the formations and developments of the electrostatic images are conducted by different means for the respective repetitions. As a result, those methods also have problems that the reproducing apparatus has its size enlarged to raise the cost, and that the synchronous control of the exposures of the respectie exposing means is so difficult as to invite the color shift. Here, the method disclosed in Japanese Patent Laid-Open No. 116554/83 is different from the method disclosed in Japanese Patent Laid-Open No. 144452 in that the respective developments by the reversal developing method are conducted under the contact developing condition, in which the developer layers formed by the developing means brush the surface of the image retainer, thereby to solve the problems of the reversal developing method that the sufficient developing density can hardly be obtained and that the toner is liable to be scattered. Moreover, the method disclosed in Japanese Patent Laid-Open No. 116553/83 is different from the same Japanese Patent Laid-Open No. 144452/81 in that, in the second and later retentions of the electrostatic images, too, the surface of the image retainer is recharged before exposure by the chargers, which are placed in front of the respective exposing means, so that toner in another color may not be applied during a later development to the portions having the toner adhered thereto after the previous development. Since the second and latter developments are conducted under the contact developing condition, however, those methods have a serious problem that the toner adhered after the previous development is liable to be shifted during the subsequent development or to be mixed into the developer of the subsequent developing means.
A prototype in which an electrostatic latent image is expressed in a multi-color image is concerned with a color image using an electrophotographic system. This system of the prior art separates the colors of an original through an optical filter and repeats the charging, exposing, developing and transferring steps by using the separated colors. In order that respective images of color particles such as yellow, magenta, cyan and black colors may be retained, more specifically, those steps are repeated four times by that system. There also exists the so-called "dichromatic developing method", in which electrostatic latent images of different polarities are formed on a common photosensitive member (or an image carrier) and are developed by particles of black and red colors. These multi-color image retaining methods are desirable, because they can add color data as compared with the data obtained from the dichromatic images, but have the following problems:
(1) Transfer to a transfer member is required at each development of each color to enlarge the size of the machine and to elongate the time period necessary for the image retention; and
(2) It is necessary to ensure the accuracy of positional shifts resulting from the repetitions.
In view of these problems, there has been conducted a trial in which a plurality of toner images are developed in a superposed manner on a common photosensitive member so that the transfer step may be finished by one time to reduce the size of the machine.
As the developer to be used in this machine, on the other hand, there exists a two-component developer, which is composed of a toner and a carrier, and a one-component developer which is composed only of a toner. The one-component developer has some problems in the charge control of the toner but has advantages that no consideration is necessary into the concentration and agitation of the toner and the carrier, and that the size of the machine can be reduced.
The two-component developer requires control of the ratio of the toner to the carrier but has an advantage that it is easy to control the frictional charges of the toner particles. Since a magnetic material of black color need not be much contained in the toner particles, on the other hand, the two-component developer composed of a magnetic carrier and a non-magnetic toner can use a color toner having no color turbidity by the magnetic material so that a clear color image can be formed.
In the multiplex development, incidentally, it is sufficient to repeat several times the developments of the photosensitive member which has already been formed with the toner image. However, the multiplex development has problems that the toner image retained at a previous step on the photosensitive member is disturbed upon development of a subsequent step, and that the toner having already been applied to the photosensitive member is returned to a developing sleeve acting as a developer carrier until it steals into the developing means at a subsequent step, in which a developer in a color different from that of the developer of the previous step, thereby to cause color mixing. In order to obviate those problems, there is disclosed in Japanese Patent Laid-Open No. 144452/81, for example, means for superposing an a.c. component upon a developing bias while the photosensitive member being out of contact with the developer layer on the developing sleeve acting as the developer carrier for developing an electrostatic latent image, except the developing means for first forming the toner image on the photosensitive member. However, there arises a problem that the image can neither have a sufficient density nor be freed from the disturbance or color mixing.